Vehicles of the type in which the front wheels are driven by an internal combustion engine mounted in a front portion of the vehicle body, unlike rear-drive type vehicles, do not need a propeller shaft for transmitting the drive force of the engine to the rear wheels, and therefore in such front-drive type vehicles a center tunnel is not needed for the purpose of accommodating the propeller shaft. However, it has been a common practice to adopt a structure having a center tunnel in order to ensure sufficient rigidity of the floor structure, although it is not favorable in view of improving amenity of a passenger compartment. As a structure based on such view, a floor structure is known in which a flat floor panel is used while the rigidity of the floor structure is ensured by a cross member connecting the left and right side sills. In such a floor structure, the cross member is usually provided on the under surface of the floor panel to achieve a flat floor surface. However, the cross member under the floor panel interferes with external piping such as an exhaust pipe, and to avoid the interference the piping must be vertically overlapped with the cross member. This results in a higher position of the floor, which is not preferable in view of easy getting in and out of the vehicle.
Further, in the floor structure having a center tunnel, a muffler for deadening exhaust noises which is relatively bulky in the vertical direction can be accommodated in the center tunnel, but with a flat floor panel, it has been necessary to increase the height of the floor in order to mount the muffler under the floor panel.
Thus, it has been difficult to achieve a flat floor surface and at the same time lower the position of the floor in a favorable manner.